vc/google/chromeos: load wifi_sar_defaults.hex as the main WiFi SAR CBFS source

Each variant WiFi SAR CBFS will be added with the default name
"wifi_sar_defaults.hex".
so we just need to look up the default CBFS file as the WiFi SAR
source.

BUG=b:159304570
BRANCH=zork
TEST=1. cros-workon-zork start coreboot-private-files-zork
     2. emerge-zork chromeos-config coreboot-private-files-zork \
        coreboot chromeos-bootimage

Change-Id: Idf859c7bdeb1f41b5144663ba1762e560dcfc789
Signed-off-by: Kevin Chiu <kevin.chiu@quantatw.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44672
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/vendorcode/google/chromeos/sar.c

@@ -113,5 +113,5 @@ int get_wifi_sar_limits(struct wifi_sar_limits *sar_limits)
 __weak
 const char *get_wifi_sar_cbfs_filename(void)
 {
-	return NULL;
+	return WIFI_SAR_CBFS_FILENAME;
 }